Factors to Determine Inclusion Compositions in Molten Steel during the Secondary Refining Process of Case-Hardening Steel

摘要

A study was carried out to observe inclusions during the secondary refining process of case-hardening steel to understand the factors to determine inclusion compositions. During the LF refining, inclusions changed from the primary deoxidation product of Al_(2)O_(3) to MgO·Al_(2)O_(3) and the CaO–Al_(2)O_(3) system. At the end of LF refining, the compositions were placed on the tie-line connecting the areas of MgO·Al_(2)O_(3) and CaO–Al_(2)O_(3)–MgO_(liq) in the CaO–Al_(2)O_(3)–MgO diagram. This change took place by the composition evolution targeting the thermodynamic equilibrium states. After the RH treatment, the inclusion compositions changed mainly to Al_(2)O_(3) and the CaO–Al_(2)O_(3) system because MgO·Al_(2)O_(3) inclusions were removed, while the CaO–Al_(2)O_(3) system inclusions, the most stable oxide, were remained. This behavior could be described by the interfacial properties between the oxides and the molten steel. Al_(2)O_(3) inclusions were considered to be newly generated during the RH treatment. It was confirmed that three factors of (1) equilibrium state (2) removal and (3) generation of inclusions dominated to determine the inclusion compositions.